Air is a mixture of gases approximately composed of 78.08% nitrogen (N2), 20.95% oxygen (O2), 0.93% argon (Ar), 0.038% carbon dioxide (CO2), trace amounts of other gases, and a variable amount (average around 1%) of water vapor. At ambient temperatures, the oxygen and nitrogen gases in air will not react with each other. However, in an internal combustion engine, combustion of a mixture of air and fuel produces combustion temperatures high enough to drive endothermic reactions between atmospheric nitrogen and oxygen in the flame, yielding various oxides of nitrogen, such as nitric oxide (NO) and nitrogen dioxide (NO2). Mono-nitrogen oxides such as NO and NO2 are typically referred to by the generic term NOx.
Nitrogen dioxide (NO2) is a major pollutant in the atmosphere of modern cities that is easily recognized by its reddish brown color. NO2 is formed when nitric oxide (NO) is produced as a byproduct of combustion in internal combustion engines and power generators at temperatures greater than 800° C. and is oxidized by alkyl peroxy radicals in the atmosphere. In California and much of the United States, a principal source of NO is from trucks, since auto emissions have been successfully reduced by use of catalytic converters. NO2 in the troposphere subsequently undergoes photolysis to ultimately form ozone (O3) in the presence of sunlight. In the stratosphere, however, NO2 is implicated in the destruction of O3. Mixing ratios for NO2 have been measured at sub-parts-per-billion levels in remote areas and up to hundreds of parts per billion (ppb) in urban areas.
Although techniques have been developed to measure atmospheric NO2, these techniques have deficiencies related to interferences, stability and precision. As a result, the measured atmospheric NO2 may not be accurate. Consequently, a technique to measure atmospheric NO and NO2 that lacks interference and is more stable and precise is needed.